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What is a ganglion?

Definition, anatomy and function of a ganglion.

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Hey everyone! This is Nicole from Kenhub, and in this tutorial, we'll be answering the question, what is a ganglion? Before we get into the definition of a ganglion, let's just remind ourselves of how we divide up the nervous system. Structurally, the nervous system can be divided into two parts – the central nervous system or the CNS which contains the nerve cell bodies of the brain and the spinal cord and the peripheral nervous system or the PNS which are the nerve cell bodies of everything that is not contained within the CNS.

The nervous system is made up of structural and functional units called neurons which in turn are made up of cell bodies and processes. A group of nerve cell bodies within the CNS is called a nucleus like this cross-sectional image of the nuclei of the solitary tract while a group of nerve cell bodies within the PNS is called a ganglion like this image of the cervicothoracic ganglion highlighted in green. Therefore, a ganglion can be defined as a cluster of nerve cell bodies located in the peripheral nervous system. The pleural of ganglion is ganglia.

The nervous system can be broken down functionally into the somatic nervous system or the SNS which broadly controls the skeletal muscles and voluntary movement and the autonomic nervous system, the ANS, which broadly controls the viscera and involuntary movement. Both the somatic and autonomic systems have sensory and motor fibers and this is an important distinction when it comes to the different types of ganglia – that is nerve cell bodies found in the peripheral nervous system – of which there are two main types: the sensory ganglia which is any type of ganglion sending sensory impulses from the PNS such as this image of the sensory ganglia of the spinal nerves and the autonomic motor ganglia which is any type of ganglion sending involuntary motor movements within the viscera such as this image of the superior cervical ganglion. And we'll talk a little bit more about each in the following slides.

The sensory ganglia are the ganglia in the peripheral nervous system that receives sensory or afferent information from the periphery before transmitting that information to either the spinal cord or the brain. The sensory ganglia which contain the cell bodies of sensory neurons can be further divided into the dorsal root ganglia and some of the cranial nerve ganglia. Let's talk first about the dorsal root ganglia.

The dorsal root ganglia also known as the posterior root ganglia is a cluster of cell bodies that bring sensory information from the peripheral body to the spinal cord. It is found on the dorsal root of a spinal nerve and in the image we can see it highlighted in green. The dorsal root ganglia are situated in the intervertebral foramina and as you can see the ventral root of the spinal nerve does not possess a ganglion.

The sensory cranial nerve ganglia are clusters of cell bodies that receive sensory information from the periphery and then transmit that information to the respective receptors in the brain. And there are six of these which include the trigeminal ganglion or the sensory ganglion of the fifth cranial nerve and this is also known as the Gasserian or semilunar ganglion, the geniculate ganglion of the facial nerve which is the ganglion of the seventh cranial nerve; the spiral ganglion of the vestibulocochlear nerve or the ganglion of the eighth cranial nerve and this ganglion also contains bipolar neurons; the vestibulocochlear ganglion of the vestibulocochlear nerve is also a ganglion of the eighth cranial nerve and is sometimes called Scarpa's ganglion – it also contains bipolar neurons and as you can see it has a superior part and an inferior part; and continuing our list on the next slide, we have the glossopharyngeal nerve ganglia which are the ganglia of the ninth cranial nerve and the glossopharyngeal nerve ganglia has a superior ganglion and an inferior ganglion; and finally the vagus nerve ganglia are the ganglia of the tenth cranial nerve and similarly it has a superior ganglion and an inferior ganglion.

The second type of ganglia that are found in the PNS – the autonomic ganglia – are postsynaptic or efferent ganglia of the autonomic nervous system. And these can be divided into the sympathetic ganglia and the parasympathetic ganglia which includes the rest of the cranial nerve ganglia although there are others which we won't talk about today called terminal ganglia and intramural ganglia that synapse close to their respective organs. And in this tutorial, we'll only look at the sympathetic and parasympathetic ganglia very briefly. Let's firstly have a look at the sympathetic ganglia.

The sympathetic ganglia are ganglia of the sympathetic nervous system and the sympathetic ganglia include the paravertebral ganglia which form two chains of sympathetic trunks on either side of the vertebral column, and in this image we're looking at the paravertebral trunks of the thorax. The prevertebral ganglia which are found within the plexuses surrounding the branches of the abdominal aorta such as this superior mesenteric plexus beneath the celiac trunk and the adrenal or suprarenal medulla which when stimulated releases adrenaline into the bloodstream. As the medulla is the innermost part of the adrenal gland, you won't be able to see it in this image.

The parasympathetic ganglia are ganglia of the parasympathetic nervous system. The parasympathetic ganglia of the cranial nerve ganglia include the ciliary ganglion of the oculomotor nerve which is the ganglion of the third cranial nerve, the submandibular ganglion of the facial nerve, the pterygopalatine ganglion and the otic ganglion.

As we saw at the beginning of this tutorial, we discussed how the ganglia which are clusters of nerve cell bodies are generally found in the peripheral nervous system but there's one last group of interconnected nuclei that I want to mention and these are the basal ganglia. The basal ganglia are found deep in the cerebral cortex, the thalamus and the brainstem and on the image of the cerebral hemispheres cut in cross section on the right, we can see some of the basal ganglia highlighted in green that is the caudate nucleus, the claustrum, the putamen nucleus and the globus pallidus. The remaining nuclei of the basal ganglia are not visible at this level, and the basal ganglia have an important role in motor function mainly planning and modulation of movement pathways and cognition and emotion.

One last thing I wanted to add as we finish this tutorial is the pseudoganglion. The pseudoganglion is a thickened nerve trunk comprised of nerve fibers which gives the appearance of a ganglion but it's important to note that this is not a real ganglion and pseudoganglions can often be found in the nerve supplying the teres minor muscle highlighted in green which is a branch of the axillary nerve.

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